Silver plating method and articles made therefrom

ABSTRACT

An improved method for plating an organic substrate with silver is disclosed. Improved plating is achieved in part through the use of Na 4 EDTA, which facilitates improved grain formation and recovery of silver from waste material. Articles prepared using the method of the invention are also disclosed.

[0001] This application claims priority under 35 U.S.C. §119(e) fromfollowing Provisional Applications, which are herein incorporated byreference: 60/412,301; 60/412,302; 60/412,303; and 60/412,306 all filedon Sep. 20, 2002.

FIELD OF THE INVENTION

[0002] The present invention relates to an improved, electroless silverplating method particularly suitable for the production of articleshaving antimicrobial and anti-static properties.

BACKGROUND OF THE INVENTION

[0003] Metallization of organic substrates (e.g., polymeric materials)with silver and other noble metals is well known in the art. One suchtechnique is described in U.S. Pat. No. 3,877,965 to Broadbent et al.,which describes metallizing nylon substrates with silver and isincorporated herein by reference. Articles metallized with silver havefound a wide variety of uses due to the inherent antimicrobial andanti-static properties of silver. For example, silver plated nylonfibers are commonly woven into textile materials which in turn are usedfor consumer products (e.g., socks, wound dressings) and forelectromagnetic interference (EMI) shielding applications for electronicequipment (e.g., cellular telephones, computers).

[0004] However, current processes for metallizing substrates with silverdo have certain disadvantages. For example, the process described inU.S. Pat. No: 3,877,965 has been found to exhibit several disadvantages.First, with most metallization processes, the pre-metallization stepsrequire the tin salt (e.g., stannous or stannic chloride) to bedissolved with the aid of a water-soluble alcohol (e.g., a C₁-C₄alcohol). The use of an alcohol results in significant evaporationproblems.

[0005] A more significant problem is the use of a surfactant (e.g.,sodium lauryl sulfate) during the metallization process. For example,the surfactant can lead to gelling of the plating bath if the bathtemperature is too low. Likewise, the surfactant can cause significantfoaming in the plating bath, which is difficult to remove aftermetallization is completed. Typically, foam generated during themetallization process ends up on the surface of the fibers. Once on thefiber surface, the foam becomes difficult to rinse off properly. This inturn potentially results in inhibiting silver ion release and alsopresent adhesion issues due to the surface cracking when exposed to hightemperatures. Cracking occurs as the contaminants (e.g., entrained air)are forced out under pressure from beneath the surface of the silverlayer.

[0006] In addition, the use of a surfactant results in the color of thedeposited silver being a very dark gray/silver (and sometimes evenbrown). This alternative color of the deposited silver lowers theaesthetic value of the product since consumers normally expectmetallized silver to exhibit a bright metallic color.

[0007] However, one of the most significant problems associated with theuse of a surfactant is the environmental impact and the associated costsof removing the surfactant from the waste effluent. Local sewerauthorities, the Environmental Protection Agency (EPA) and other similarorganizations now mandate very low discharge levels of “Methylene BlueActivated Substance” (i.e., MBAS). Surfactants such as sodium laurylsulfate fall within the category of MBAS. In order to reduce MBAS toacceptable limits (typically 5 parts per million (ppm) or lower),significant effort and expense are required on waste treatment which inturn increase productions costs and time. Typically, removal of MBASrequires the use activated charcoal chemistries along with high-gradeadsorbent (activated charcoal) filters, which must be changed frequentlydue to clogging by particulate. Likewise, the amount of silver recoveredfrom the waste is problematic because the recovered silver is in a lowconcentration as a percentage of the amount of sludge (e.g., aroundabout 1 percent by weight).

[0008] In view of the disadvantages associated with surfactants, othermetallization processes have been developed that use non-surfactantbaths. Typically, these processes employ disodiumethylenediaminetetraacetic acid (EDTA) as a ligand instead of asurfactant. Representative patents relating to the use of disodium EDTAin silver plating are U.S. Pat. Nos. 5,318,621 and 5,322,553, bothassigned Applied Electroless Concepts. Another example is U.S. Pat. No.5,158,604 assigned to Monsanto Company. However, this technology is notwithout problems since high amounts of caustic soda must be used todissolve and adjust pH of the plating bath.

SUMMARY OF THE INVENTION

[0009] The present invention advantageously provides an improved methodfor plating an organic substrate with silver that avoids many of thedisadvantages associated with prior silver plating methods. The methodof the invention entails at least three (3) steps followed in sequence:(a) scouring; (b) pre-metallization; and (c) plating. Organic substratesto be plated can be in the form of fibers, a textile woven from fibers,or a polymeric foam (e.g., an open cell foam). In accordance withinvention, the organic substrate is first scoured to prepare the surfacefor pre-metallization. Preferably, an aqueous cleaning solution is used.

[0010] Once the organic substrate has been sufficiently cleaned, thescoured, organic substrate is contacted with an aqueous,pre-metallization solution including a tin salt and an inorganic acid.In one embodiment, pre-metallization solution omits a water-soluble orwater-miscible solvent. In another embodiment, the pre-metallizationsolution omits a surfactant. Tin salts to be used include stannouschloride, stannic chloride, and mixtures thereof. Inorganic acids to beused include hydrochloric acid, sulfuric acid, and mixtures thereof.

[0011] The pre-metallized, organic substrate is thereafter plated withsilver, which comprises:(i) contacting the pre-metallized, organicsubstrate with an aqueous Na4EDTA solution;(ii) subsequently contactingthe pre-metallized, organic substrate with an additional aqueous, silversalt solution to effect deposition of a silver oxide on the organicsubstrate, wherein the silver salt solution further includes acomplexing agent; and (iii) contacting the organic substrate having thedeposited silver oxide with a reducing agent thereby effecting formationof metallic silver on the organic substrate. Particularly preferredsilver salts and complexing agents are silver nitrate and aqueousammonia, respectively. A preferred class of reducing agents is reducingagents including an aldehyde functional group. Representative examplesof reducing agents include formaldehyde, rochelle salts (sodiumpotassium tartrate), hydrazine, dextrose, triethanol amine, glyoxal,inverted sugar, glucose, sodium borohydride, dimethyl amineborane,hydrazine borane and mixtures thereof. In another embodiment, all of thesolutions in the plating step preferably omit a surfactant

[0012] The present invention also provides articles prepared inaccordance with the method of the invention. In one embodiment, theorganic substrate further includes at least one layer of a non-noblemetal disposed thereon, and is preferably disposed on the platedmetallic silver layer. One particularly preferred non-noble metal iscopper. In accordance with the invention, the metallic silver layer isleast 5 percent by weight of the article, with at least 10 percent byweight being more preferred.

[0013] Advantageously, the method of the invention allows the use ofsurfactants to be omitted while increasing the recovery of silver fromwaste products. Thus, environmental concerns can be alleviated throughthe use of the invention as compared to prior processes.

BRIEF DESCRIPTION OF THE DRAWING

[0014]FIG. 1 is an electron micrograph at 960× magnification of nylonfibers plated with the silver using the method of the invention.

[0015]FIG. 2 is an electron micrograph at 5000× magnification of nylonfibers plated with the silver using a prior art process.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present invention provides an improved method for plating anorganic substrate with metallic silver while avoiding many of thedisadvantages associated with the prior art. The method of the inventionentails first scouring the organic substrate to prepare the surface forpre-metallization. Once the organic substrate has been sufficientlyscoured, the organic substrate is contacted with an aqueous,pre-metallization solution including a tin salt and an inorganic acid.Plating is thereafter accomplished by contacting the pre-metallized,organic substrate with an aqueous Na₄EDTA solution that in turn isfollowed by contacting the pre-metallized, organic substrate with anaqueous, silver salt solution to effect deposition of a silver oxide onthe organic substrate. The silver salt solution further includes a knowncomplexing agent. The deposited silver oxide is converted (i.e.,reduced) to metallic silver by contacting the organic substrate with areducing agent thereby effecting formation of metallic silver.

[0017] In accordance with the invention, organic substrates to bemetallized with silver include any organic material capable of receivinga deposited metallic layer. The organic material can be synthetic ornatural with synthetic (e.g., polymeric) materials being preferred.Examples of synthetic polymeric materials to be used include, but arenot limited to, nylon, polyester, acrylic, rayon, and polyurethane.Examples of natural materials include, but are not limited to,cellulose, and silk. The organic materials can be in any physical formcapable of receiving the deposited metallic layer. For example, theorganic material can be in the form of filaments, fabrics, staple,chopped fibers, micronized fiber, foams, particulates and fillermaterials. Preferably, the organic material is in the form of a fiber orfilament, or a textile matrix made therefrom. If the organic material isin the form of a foam, an open-cell foam (i.e., has a three-dimensionalinterconnected network of cells) is preferred to allow metallizationthroughout.

[0018] The organic substrate is first prepared for pre-metallization byscouring to remove debris and/or to remove any coatings or film on thematerial that may interfere with metallization. Scouring is a techniquewell known in the art and thus does not require much discussion.Typically, the material is washed with an aqueous cleaning solution thatmay or may not contain a surfactant (e.g., a nonionic surfactant). Inaccordance with the invention, reference to “aqueous” means at least amajority of the medium is water with the remaining portion being awater-soluble or water-miscible organic solvent. The organic materialcan also be abraded using a scouring brush or equivalent device. In apreferred embodiment, scouring is accomplished with a high-speed waterspray, which facilitates in-line processing and avoids the necessity ofa scouring brush.

[0019] Once the organic substrate has been sufficiently scoured, thematerial is subjected to pre-metallization with an aqueous solution of atin salt and an inorganic acid. As will be apparent to one skilled inthe art, such a solution is often referred to as a “sensitizing”solution. However, unlike prior art “sensitizing” solutions, thepre-metallization solution preferably omits a surfactant and/or awater-soluble or water-miscible organic solvent such as a C₁-C₄ alcohol.Preferably, the tin salt is a halide such as stannous chloride, stannicchloride, or mixtures thereof. Examples of inorganic acids include, butare not limited to, hydrochloric acid, sulfuric acid, and mixturesthereof. In a more preferred embodiment, the tin salt is stannouschloride and the inorganic acid is hydrochloric acid. Ranges of the twocomponents are set forth in Table 1: TABLE 1 Tin Salt Inorganic Acid(grams/liter) (% by Volume) Preferred 1-30 1-20 More Preferred 2-25 3-18Optimal 4-20 6-15

[0020] Once the organic substrate has been pre-metallized, the organicsubstrate is preferably washed to remove excess salt and acid from theorganic substrate that can interfere with subsequent metallization. Forexample, the organic substrate can be washed with a counter flow rinsewith controlled water flow. This enables the removal of any excess saltsand acids from the substrate material while leaving optimal amount ofactivated sites on the surface of the substrate. Preferred levels ofwater flow to wash the substrate range from about 25 to about 55 gallonsper minute (gpm), with 30 to 50 gpm being more preferred, and 35 to 45gpm being more preferred.

[0021] In accordance with the present invention, metallization isaccomplished in three (3) substeps. An aqueous tetrasodiumethylenediaminetetraacetic acid (Na₄EDTA) is prepared into which thepre-metallized organic substrate is contacted preferably by immersingthe substrate in the aqueous solution. The aqueous solution ispreferably prepared using de-ionized (DI) water to avoid possiblecontamination. The DI water should have a resistance of about 0.4 toabout 20 megaohms, with 0.8 to 10 megaohms being preferred, and 3 to 7megaohms being more preferred. The concentration of the aqueous Na₄EDTAsolution should range from about 5 to about 30 percent by weight (wt.%), with 10 to 25 wt. % being preferred, and 10 to 20 wt. % being evenmore preferred. Preferably, the Na₄EDTA solution omits a surfactant asis typically found in conventional silver plating processes. Likewise,the Na₄EDTA solution also preferably omits caustic soda as typicallyfound in Na₂EDTA solutions. Advantageously, the use of Na₄EDTAfacilitates the deposition of metallic silver with a tighter grainstructure, which in turn leads to a relatively smoother surface asevidenced by examination of silver-plated nylon fiber by electronmicroscopy. Likewise, Na₄EDTA allows a surfactant to be omitted thusalleviating environmental concerns regarding levels of surfactant in thewaste effluent.

[0022] An aqueous silver salt solution is also prepared for subsequentcontacting of the organic substrate. Preferably, the organic substrateis contacted with the silver salt solution by adding the silver saltsolution directly to the bath containing the organic substrate and theaqueous Na₄EDTA solution. Thus, the organic substrate iscontemporaneously immersed in both solutions, which is referred to asthe “metallization bath.” Alternatively, the organic substrate can beremoved from the Na₄EDTA solution and subsequently immersed in thesilver salt solution. One particularly preferred silver salt is silvernitrate (i.e., AgNO₃). The silver salt solution additionally includes acomplexing agent as known in the art, which form a complex in situ withthe dissolved silver salt. One particularly preferred complexing agentis aqueous or aqua ammonia (i.e., NH₄OH) which is commonly used as acomplexing agent for silver nitrate. As with the Na₄EDTA solution, thesilver salt solution preferably omits a surfactant.

[0023] The silver salt solution is preferably prepared by firstdissolving the silver salt in water. Once the silver salt has beendissolved, the complexing agent is added to the solution. A precipitateof a silver oxide can form and is re-dissolved through the addition ofexcess complexing agent. The addition of excess complexing agent isbelieved to form a complex of the silver salt and the complexing agent.For example, when silver nitrate and aqua ammonia are used, aprecipitate of silver oxide forms in situ which is re-dissolved upon theaddition of excess aqua ammonia to provide a metallization bath having alight amber color. Preferred initial weight/volume ratios of silver salt(i.e., AgX) to water (H₂O) and of percent by volume of complexing agentare set forth in Table 2. Preferred molar ratios of silver salt tocomplexing agent in the final metallization bath (i.e., uponre-dissolution of the silver precipitate) are set forth in Table 3.TABLE 2 AgX:H₂O Complexing Agent (weight:volume) (percent by volume)Preferred 0.25:2 to 1.75:2 17 to 38 More Preferred 0.5:2 to 1.5:2 20 to35 Optimal 0.75:2 to 1.25:2 25 to 31

[0024] TABLE 3 Molar Ratio of Complexing Agent:AgX Preferred 2.5:1 to5.5:1 More Preferred 3:1 to 5:1 Optimal 3.5:1 to 4.5:1

[0025] In accordance with the invention, immersion of the organicsubstrate in the metallization bath results in the deposition of silveroxide on the substrate surface. As will be apparent to one skilled inthe art, deposition can be confirmed by a visual inspection of thesubstrate undergoing a change in color due to the deposited silveroxide. For example, in the case of AgNO₃:NH₃ one will typically observethe substrate to develop a shade of brown on the surface whichcorrelating to a silver oxide layer. Preferably, the organic substrateis immersed in the metallization bath prior to the addition of thereducing agent for about 30 seconds, with 20 seconds being morepreferred. The temperature of the metallization bath is not critical andcan range from about 15 to about 45° C., with 20 to 30° C. being morepreferred. These parameters can be easily determined by one skilled inthe art.

[0026] The organic substrate with the silver oxide thereon issubsequently contacted with a reducing agent to convert the silver oxideto metallic silver. Preferably, contacting is accomplished by adding thereducing agent directly to the metallization bath. Alternatively, theorganic substrate is removed from the metallization bath and isseparately contacted (e.g., immersed) with an aqueous solution of thereducing agent. Reducing agents to be used in accordance with theinvention are well known in the art. Examples of reducing agents to beused include, but are not limited to, formaldehyde, rochelle salts(sodium potassium tartrate), hydrazine, dextrose, triethanol amine,glyoxal, inverted sugar, glucose, sodium borohydride, dimethylamineborane, hydrazine borane. More preferred are reducing agentscontaining an aldehyde functional group such as formaldehyde. In thecase where AgNO₃:NH₃ is used, the addition of the reducing agent (e.g.,formaldehyde) results in the silver oxide layer on the substratechanging color to a bright gold or gray-gold color as the silver oxideis converted to metallic silver. Preferably, the amount of reducingagent to be used ranges from about 5 to about 40 percent by weight ofsubstrate, with 6 to 25 percent by weight being preferred, and 8 to 22percent by weight being more preferred.

[0027] Once sufficient metallic silver has been converted, the organicsubstrate is removed from the metallization bath and washed. Preferably,the silver-plated substrate is immersed in hot water. The silver-platedsubstrate is then preferably immersed in a weak solution of sodiumhydroxide, which brightens the silver plating to a light gold color or alight gray color. This indicates that a pure layer of silver depositedon the substrate. The article can be subjected to multiple rinse cyclesto ensure the cleanliness of product.

[0028] As will be apparent to one skilled in the art, the amount ofmetallic silver deposited on the organic substrate is a function ofimmersion time. In accordance with the invention, the time for completedeposition of the metallic silver layer will be less than 4 hours.However, time periods for immersing the substrate in the varioussolutions can easily be altered depending on the amount of depositedsilver desired. The amount of silver deposited on the organic substratecan range from 0.1% to 15% by weight, depending on the specificcharacteristics desired for the final product. Preferably, the depositedsilver layer is at least 5 percent by weight, with at least 10 percentby weight being more preferred. The actual amount of silver deposited onthe substrate is easily calculated by a simple titration such as theVollard process.

[0029] Unlike the prior art process of U.S. Pat. No: 3,877,965, theremaining metallization bath is treated with ease. The pH of themetallization bath is raised to approximately 13 which results in thedissolved silver being converted into a colloidal form. The pH is thenlowered to around 2 typically with the help of an inorganic acid (e.g.,sulfuric acid). The colloidal silver precipitates and settles down atthe bottom of the container after several hours. The purity of thesilver precipitated has been found to be up to 50 percent by weight.

[0030] As will be apparent to one skilled in the art, the adhesion ofthe plated silver is easily ascertained. One simple test for adhesion ofthe silver to the substrate requires placing a sample into an oven at200° C. for about 5 minutes and then boiling the same sample for 1 hourin water. The resistance of the sample before and after heating andboiling are compared. A variation in resistance of no more than about 20percent indicates excellent adhesion. In a more preferred embodiment,the variation of resistance is no more than 10 percent.

[0031] In another embodiment of the invention, the silver-platedsubstrate is additionally plated with a non-noble metal such as copperas described in U.S. Pat. No. 3,877,965. Copper is auto-catalytic onsilver and thus can reduce itself easily for form a copper layer. Usingsuch process up to 30% by weight of copper is deposited on to thesilver-plated substrate. Commercial plating solutions are available fromAtotech USA, Enthone OMI, and MacDennid Corporation.

[0032] The following non-limiting examples illustrate the use of themethod of the invention to plate organic substrates with metallic silverlayers.

EXAMPLES Example 1

[0033] A 30/10 knit sample of nylon weighing 25 grams was scoured toremove any contaminants. The knit sample was wrapped into a skein andscoured in counter flow de-ionized water. The sample was pre-metallizedwith a solution containing 1% by volume HCL and 10 grams of anhydroustin chloride (SnCl₂) for about 2 minutes. A silver salt solution wasprepared by dissolving 0.04 grams of silver nitrate (0.1% silver byweight target) in de-ionized water. The silver salt was then complexedwith 0.045 mL of 27% by volume aqua ammonia. A tetrasodium EDTA solutionwas prepared by dissolving 0.002 grams Na₄EDTA in 1 liter of de-ionizedwater. The skein was placed in the reactor containing the Na₄EDTAsolution and made to revolve. The silver salt solution (i.e., complexedsilver nitrate and ammonia) was added to the reactor slowly until allcontents were emptied. This was followed by 0.016 mL formaldehyde. Afterthree hours the sample was removed and subjected to hot water rinse. A0.1% by volume NaOH solution (1 liter) was prepared with a temperatureof 70° C. The metallized skein was then dipped into the solution andrinsed thoroughly. The sample was subjected to Dow Corning CorporateTest Method 0923: organism —Staphylocococcus aureaus ATCC 7538; samplesize—0.75 grams; results—percent reduction in colony>99.9%.

Example 2

[0034] As in example 1, a 30/10 knit sample of nylon weighing 25 gramswas scoured to remove any contaminants. The knit sample was wrapped intoa skein and scoured in counter flow de-ionized water. The sample waspre-metallized with a solution containing 1% by volume HCL and 10 gramsof anhydrous tin chloride (SnCl₂) for about 2 minutes. A silver saltsolution was prepared by dissolving 1.95 grams of silver nitrate (5%silver by weight target) in de-ionized water. The silver salt was thencomplexed with 2.25 ml of 27% by volume aqua ammonia. A tetrasodium EDTAsolution was prepared by dissolving 0.1 grams of Na₄EDTA in 1 liter ofde-ionized water. Skein was placed in the reactor containing the Na₄EDTAsolution and made to revolve. The silver salt solution (i.e., complexedsilver nitrate and ammonia) was added to the reactor followed by 0.8 mLof formaldehyde. After three hours the sample was removed and subjectedto hot water rinse. The metallized skein was rinsed in a NaOH solutionas in Example 1. The sample was subjected to Dow Coming Corporate TestMethod 0923: organism—Staphylocococcus aureaus ATCC 7538; samplesize—0.75 grams; results—percent reduction in colony>99.9%.

Example 3

[0035] A 25 grams sample of ripstop fabric was processed following theprocedure of examples 1 and 2. The silver-plated sample was thensubjected to Dow Corning Corporate Test Method 0923:organism—Staphylocococcus aureaus ATCC 7538; sample size—0.75 grams;results—percent reduction in colony>99.9%.

Example 4

[0036] A 25 gram sample of filler material including nano powders (i.e.,powder made from nylon and polyethylene) was processed following theprocedure of examples 1 and 2. The silver-plated sample was thensubjected to Dow Corning Corporate Test Method 0923:organism—Staphylocococcus aureaus ATCC 7538; sample size—0.75 grams;results—percent reduction in colony>99.9%.

Example 5

[0037] A 30/10 knit sample of nylon weighing 118 grams was scoured toremove any contaminants. The knit sample was wrapped into a skein andscoured in counter flow de-ionized water. The sample was pre-metallizedwith a solution containing 10% by volume HCL and 100 grams of anhydroustin chloride (SnCl₂) for about 2 minutes. A silver salt solution wasprepared by dissolving 45 grams of silver nitrate (about 22% silver byweight target) in de-ionized water. The silver salt was then complexedwith 52 mL of 27% by volume aqua ammonia. A tetrasodium EDTA solutionwas prepared by dissolving 2.2 grams Na₄EDTA in 6 liters of de-ionizedwater. The skein was placed in the reactor containing the Na₄EDTAsolution and made to revolve. The silver salt solution (i.e., complexedsilver nitrate and ammonia) was added to the reactor slowly until allcontents were emptied. This was followed by 18 mL of formaldehyde. Afterthree hours the sample was removed and subjected to hot water rinse. A0.1% by volume NaOH solution (5 liters) was prepared with a temperatureof 70° C. The metallized skein was then dipped into the solution andrinsed thoroughly. The color changed to light almost gold coloredsilver. The sample was dried and then sent for an adhesion check. Theresults were as follows: as is—484 Ohms (50 cm distance) using aKeithley 580 micro-ohmmeter; after heat—345 Ohms; and after boil—365Ohms.

Example 6

[0038] A sample obtained from the silver-plated materials from example 5was cut to make a 1.5 gram sleeve. The sleeve was then placed in abeaker with 5% by volume sodium chloride solution for a 24-hour period.The solution after the 24-hour period was then tested for silver ionsusing a Perkin Elmer Analyst 300. The same test was repeated over aperiod of 7 days. The release of ions was consistent each day at 0.5 ppmillustrating the sustained release of silver prepared in accordance withthe invention.

Example 7

[0039] A sample obtained from the silver-plated materials from example 5was cut to weigh 0.75 grams and subjected to Dow Coming Corporate TestMethod 0923. Organism used was Staphylococcus aureus ATCC 6538. Thesample reduced organism growth by over 99.9%.

Example 8

[0040] A 210/34 knit nylon sample weighing 118 grams was cleaned. Thesample was wrapped into a skein and scoured with a counter flow ofde-ionized water. The skein was pre-metallized in a solution of 10% byvolume HCL and 100 grams of anhydrous tin chloride (SnCl₂) for 2minutes. A silver salt solution was prepared by dissolving 45 grams ofsilver nitrate in de-ionized water. The silver salt was then complexedwith 52 ml of 27% by volume aqua ammonia. A tetrasodium EDTA solutionwas prepared by dissolving 2.2 grams of Na₄EDTA in 6 liters ofde-ionized water. Skein was placed in the reactor containing the Na₄EDTAsolution and made to revolve. The silver salt complex was added to thereactor and followed by 18 mL of formaldehyde. After three hours thesample was removed and subjected to hot water rinse. As in the previousexamples, a 0.1% by volume NaOH solution was prepared and the metallizedskein was dipped into the solution. The color changed from grey to alight almost gold colored silver.

[0041] The silver-plated sample was then metallized with commerciallyavailable copper chemistry from Atotech USA. The metallization processwas carried out following the instructions suggested by supplier.Completion of the deposition of copper can be visually determined whenthe bath changes color from a deep blue to colorless, which indicates acomplete reduction of the metal.

[0042] A 10.6 grams sample of the silver-copper material was then cutand placed in a beaker filled with 2.1 grams of Rochelle salt (i.e.,sodium potassium tartrate) dissolved with de-ionized water. A silversalt complex made up of 3.6 grams of silver nitrate and 4.3 mL of aquaammonia was then poured into the sample under constant agitation. Thepink color of silver-copper changed to a light brown at this time. A fewdrops of further diluted aqua ammonia were added drop wise into the bathwith an ink dropper under constant agitation. The color of the samplethen started to change to a dull white and eventually a bright whitecolor. This step took about 35 minutes to complete. At that time sampleis removed from bath and rinsed thoroughly with de-ionized water for 15minutes. Silver was calculated to provide a 15% by weight gain. Theresistance of the material after deposition of each metallic layer wasalso recorded; resistance with Silver—80 Ohms/50 cm using a Keithley 580micro-ohmmeter; resistance with Silver-Copper—25 Ohms/50 cm; andresistance with Silver-Copper-Silver—18 Ohms/50 cm.

Example 9

[0043] A sample of the silver-copper-silver material from example 8 wascut into a 1.5 gram sleeve. The sleeve was then placed in a beaker with5% by volume sodium chloride solution for a 24-hour period. The solutionafter a 24-hour period was then tested for silver and copper ions usinga Perkin Elmer Analyst 300. The same test was repeated over a period of7 days. The release of ions was consistent each day at 2 ppm of silverand 3 ppm of copper.

Example 10

[0044] A sample of the silver-copper-silver material from example 8 wascut to weigh 0.75 grams and subjected to Dow Corning Corporate TestMethod 0923. Organism used was Staphylococcus aureus ATCC 6538 and thematerial caused a reduction of organism growth by over 99.9%.

Example 11

[0045] A quenched foam sample weighing 11.8 grams was cleaned withnon-ionic surfactant Triton X-100 and rinsed thoroughly. A 83% by weightsulfuric acid solution was prepared and the foam was dipped in thesolution for 25 seconds to 45 seconds to ensure proper etching of thesurface. Immediately the sample was rinsed with copious amounts ofde-ionized water. The foam was pre-metallized with solution containing12% by volume HCL and 120 grams of anhydrous tin chloride (SnCl₂) for 2minutes. The foam sample was then rinsed in counter flow de-ionizedwater. A tetrasodium EDTA solution was prepared by dissolving 0.22 gramsof Na₄EDTA was dissolved in 2 liters of de-ionized water. Thepre-metallized foam was placed in reactor containing the Na₄EDTAsolution and made to revolve. A silver salt solution was prepared bydissolving 4.5 grams of silver nitrate in de-ionized water. The silversalt solution was then complexed with 5.2 mL of 27% by volume aquaammonia. The silver salt complex was added to the reactor and followedby 18 mL of formaldehyde. After three hours the sample was removed andsubjected to hot water rinse. The metallized foam was dipped into a NaOHsolution as prepared in the previous examples. The color changed to adull, white silver. Sample was dried and evaluated for resistance. Thesilver-plated foam exhibited a resistance of 0.5 Ohms/50 cm using aKeithley 580 micro-ohmmeter.

Example 12

[0046] Samples of silver-plated fiber prepared in accordance with theinvention were compared to silver-plated fibers prepared following theprocedure set forth in U.S. Pat. No. 3,877,965. Samples of both samplesof fiber were examined with a scanning electron microscope. Aphotomicrograph of the inventive fibers is found in FIG. 1, while aphotomicrograph of the comparative fibers is found in FIG. 2. As can beclearly seen from the photomicrographs, the fibers plated with theinventive method exhibit a smoother surface as compared to fibersprepared with the prior art process. This is believed due to the tightergrain structure of the silver plating provided by the method of thepresent invention.

We claim:
 1. An improved method for plating an organic substrate withsilver, which comprises: (a) scouring the organic substrate to preparethe surface for pre-metallization; (b) contacting the scoured, organicsubstrate with an aqueous, pre-metallization solution including a tinsalt and an inorganic acid; and (c) plating the pre-metallized, organicsubstrate with silver, which comprises: (i) contacting thepre-metallized, organic substrate with an aqueous Na₄EDTA solution; (ii)subsequently contacting the pre-metallized, organic substrate with anadditional aqueous, silver salt solution to effect deposition of asilver oxide on the organic substrate, wherein the silver salt solutionfurther includes a complexing agent; and (iii) contacting the organicsubstrate having the deposited silver oxide with a reducing agentthereby effecting formation of metallic silver on the organic substrate.2. The method of claim 1, wherein the organic substrate is in the formof fibers.
 3. The method of claim 2, wherein the fibers are woven into atextile.
 4. The method of claim 1, wherein the organic substrate is inthe form of a polymeric foam.
 5. The method of claim 4, wherein thepolymeric foam is an open cell foam.
 6. The method of claim 1, whereinscouring comprises washing the organic substrate with an aqueouscleaning solution.
 7. The method of claim 1, wherein the tin salt is atin halide selected from the group consisting of stannous chloride,stannic chloride, and mixtures thereof.
 8. The method of claim 1,wherein the inorganic acid is selected from the group consisting ofhydrochloric acid, sulfuric acid, and mixtures thereof.
 9. The method ofclaim 1, wherein the silver salt is silver nitrate and the complexingagent is aqueous ammonia.
 10. The method of claim 1, wherein thereducing agent includes an aldehyde functional group.
 11. The method ofclaim 1, wherein the reducing agent is selected from the groupconsisting of formaldehyde, rochelle salts (sodium potassium tartrate),hydrazine, dextrose, triethanol amine, glyoxal, inverted sugar, glucose,sodium borohydride, dimethyl amineborane, hydrazine borane and mixturesthereof.
 12. The method of claim 1, wherein the pre-metallizationsolution omits a water-soluble or water-miscible solvent.
 13. The methodof claim 1, wherein the pre-metallization solution, the Na₄EDTAsolution, and the silver salt solution omit a surfactant.
 14. An articlehaving a layer of metallic silver thereon, the article being prepared bythe process comprising: (a) scouring the organic substrate to preparethe surface for pre-metallization; (b) contacting the scoured, organicsubstrate with an aqueous, pre-metallization solution including a tinsalt and an inorganic acid; and (c) plating the pre-metallized, organicsubstrate with silver, which comprises: (i) contacting thepre-metallized, organic substrate with an aqueous Na₄EDTA solution; (ii)subsequently contacting the pre-metallized, organic substrate with anadditional aqueous, silver salt solution to effect deposition of asilver oxide on the organic substrate, wherein the silver salt solutionfurther includes a complexing agent; and (iii) contacting the organicsubstrate having the deposited silver oxide with a reducing agentthereby effecting formation of metallic silver on the organic substrate.15. The article of claim 14, wherein the organic substrate furtherincludes at least one layer of a non-noble metal disposed thereon. 16.The article of claim 15, wherein the non-noble metal is copper.
 17. Thearticle of claim 14, wherein the non-noble metal is disposed on themetallic silver layer.
 18. The article of claim 14, wherein the metallicsilver layer comprises at least 5 percent by weight.
 19. The article ofclaim 14, wherein the metallic silver layer comprises at least 10percent by weight.